A two-stage electrohydraulic servovalve is a device for converting an electrical input into a substantially-proportional hydraulic output. Such servovalves typically have a first-stage hydraulic amplifier, and secondstage valve spool. The first-stage commonly has a torque motor arranged to produce pivotal movement of a member in response to a supplied electrical current. The hydraulic amplifier may be of the "nozzle-flapper" type (see, e.g., U.S. Pat. No. 3,023,782), the "jet pipe" type (see, e.g., U.S. Pat. No. 3,922, 955), or the "deflectable jet stream" type (see, e.g., U.S. Pat. No. 3,542,051 and 3,612,103). In any event, the hydraulic amplifier is used to prdouce a differential pressure, which is then used to selectively shift the second-stage valve spool in the appropriate direction relative to the body. It is also known to provide a mechanical feedback spring wire between the second-stage spool and the torque motor pivotal member such that spool displacement off null will be substantially proportional to the polarity and magnitude of the supplied current.
Such servovalves may be further classified by the nature of the output. For example, in a "flow control" servovalve, output flow is substantially proportional to supplied current, at constant load. In a "pressure control" servovalve, the hydraulic output is a differential pressure. Other types include "pressure-flow" (PQ) servovalves, "dynamic pressure feedback" (DPF) servovalves, static load error washout (SLEW) servovalves, and "acceleration switching" (AS) servovalves. These various types and configurations are comparatively illustrated in Technical Bulletin 103, "Transfer Functions for Moog Servovalves", Moog Inc. (1965).
Servosystems, in which such servovalves are employed, may fail by virtue of a loss or interruption of the supplied system pressure, or by virtue of an upstream electrical malfunction. For example, if there is an interruption of the supplied current (i.e., i=0), then the servovalve may return to a null condition. On the other hand, there may be an aberration in the upstream electrical system such that a maximum or saturation current (i.e., i=.+-.i.sub.max) will be supplied to the servovalve. In this case, the second-stage spool will be driven in the appropriate direction toward a hard-over position so as to produce the maximum hydraulic output. If used to control the position of an airfoil surface, for example, such a hard-over failure can have disastrous consequences.
Hence, fail-fixed servovalves have been developed, as representatively shown and described in U.S. Pat. No. 3,922,955. In this type of servovalve, the hydraulic output is substantially proportional to supplied electrical current within a particular operating range of spool displacement off null. Thus, within this range, the valve functions normally as a conventional servovalve. However, abutment stops are provided on the body to limit spool displacement in either direction, and the spool is appropriately configured to recreate null-like hydraulic conditions if the spool moves to either hard-over position.
However, even when the spool is in such a hard-over position, there are still leakage flows across the various spool lobes. Upon information and belief, persons skilled in this art have heretofore attempted to minimize such leakage flows by more accurately machining the various metering and facing surfaces of the spool and body. In some applications, it may be desirable to have zero net leakage flow with respect to a control port, in the event of a hard-over failure. In others, it may be desirable to deliberately have a positive or negative net leakage flow with respect to such port in the event of such a failure. For example, such deliberate net leakage flow may be employed to slew a fluid-powered actuator toward a predetermined position.